Systems, apparatus and methods for populating and using a pressure database to determine an altitude of a unit with an unknown altitude are presented. A pressure from one or more barometric pressure sensors linked to respective base stations are interpolated to determine a reference pressure (e.g., at sea level) at arbitrary coordinates (x,y) having pressure reading. For example, a mobile station at the arbitrary coordinates (x,y) records a pressure at the mobile station. A difference between this pressure at the mobile station and the interpolated reference pressure is determined, which may directly be interpreted as a defined altitude of the mobile station.